1. Field of the Invention
The present invention relates to a microwaveable container, and more particularly, to a container constructed of microwave-transparent material for the heating or cooking of foods through the intermediary of microwave energy, wherein the container is equipped with a temperature indicating arrangement which represents a change in the temperature prevalent within the container through a visual color change evidenced in a temperature sensitive liquid crystal film.
In recent years, there has been a significant increase in the popularity of microwave ovens for the cooking and/or reheating of a large variety of foods in a quick and efficient manner. Microwave energy heats food directly by heating the moisture content of the food so as to, in turn, heat the food itself; and therefore, it has been necessary to develop containers or receptacles which are microwave-transparent so that the microwaves unimpededly reach the entire surface areas of the food contained within the oven. This phenomenon has led to the development of reusable microwaveable containers which have gained in popularity in serving both as food storage and cooking or heating vessels. The reusability of these containers is an important feature with regard to their practicality in the employment thereof in microwave ovens.
Microwave ovens vary in size and also in power levels such that cooking times for similar types of foods considerably differ from one oven model to another. Furthermore, different kinds of foods require varying lengths of microwave cooking time in order for the foods to reach to desired degree of doneness or heating in relation to their serving temperature. Presently, a consumer cooking, heating, defrosting or reheating foods in a microwave oven must rely on a trial-and-error method to determine whether or not the food is done to the desired temperature; and possibly stop the oven operating and either taste or feel the food to generally ascertain the temperature level thereof. Because some foods; for example, such as popcorn, heat very efficiently and rapidly in a microwave oven, it is very easy to unintentionally overheat or overcook them.
Visual indications of heat, such as steam or bubbling of liquids due to excessive temperatures, are usually not efficient indicators of cooking time in a microwave oven, since microwave ovens are equipped with microwave energy reflecting screens which ordinarily cover the viewing window on the oven door so as to prevent any leakage of radiation. The screens form a mesh which obscures any detailed viewing of the food chamber within the microwave oven and; consequently, the oven must be opened and the food removed for visual or physical inspection to determine the doneness thereof. This leads to a very inefficient heating or cooking of the food, since the excitation of the moisture molecules within the food must be repeated once the food is returned to the oven in order to continue heating or cooking. Although various measures have been undertaken in the microwave cooking technology in order to provide a visual indication of cooking temperature and/or cooking doneness, such visual indicators are deficient; for example, due to a lack of reusability, inconvenient positioning in relation to the food, as well as contributing to the overall cost of the microwave oven, thereby rendering these devices expensive and impractical to use.
2. Discussion of the Prior Art
In the prior art, there are disclosed several types of temperature sensitive indicators for completion of cooking or heating cycles for use in microwave ovens; however, wherein the indicators are inconvenient in use and are subject to drawbacks which can adversely affect the cooking time, as well as substantially increasing the cost of the oven.
Wong, et al. U.S. Pat. No. 4,065,655 disclose a microwave leakage indicator for microwave ovens, wherein the indicator consists of a strip of encapsulated liquid-crystal film which is mounted on a backing constituted from microwave-absorbing material. The strip of liquid-crystal film and the microwave-absorbing material are positioned on and secured to the edge of the microwave oven door so as to overlap the slot between the door and the frame of the oven. In the presence of microwaves, i.e. in the event of any leakage, the microwave-absorbing material generates heat which quickly changes the color of the liquid crystal film material to indicate the existence of leakage of harmful radiation. This device is not employable as a temperature indicator or sensor inside the cooking chamber of the microwave oven, since the presence of microwaves triggers the heat generation in the microwave-absorbing material to change the color of the liquid crystal film. This device would not be useful as a temperature indicating means, because the color change would occur as soon as the oven is turned on and there are present microwaves.
Hornung U.S. Pat. No. 3,991,615 discloses a coaxial cable-connected temperature-sensing probe which is embedded into the food load in order to measure the internal temperature of the food. The coaxial cable connects to a jack on the wall of the cooking chamber of the oven, and circuitry is provided to turn the oven off when the probe reaches a preset temperature which is sensed by thermistors arranged within the tip of the probe. Wiring is provided to connect the thermistor to the coaxial cable and then to the circuitry which controls the oven. A reflector-type handle is provided to ensure that the probe does not disrupt the microwave pattern within the cooking chamber. A temperature indicating means of this type is subject to the drawback that proper temperature indication can only be achieved for food loads which must be penetrated by the probe. Accordingly, this device is inconvenient for the measurements of the temperature of liquids, dispersed items such as fruits and vegetables, or for the reheating of various foods enclosed in a single or closed container. Additionally, this probe and its associated circuitry will, of necessity, increase the overall cost of the microwave oven.
Staats U.S. Pat. No. 4,341,937 describes a microwave oven having a support shelf which supports a food load at an intermediate region or raised level within the cooking cavity of the oven. An electromagnetic field sensor is located opposite a probe antenna such that the intermediate region where the food load is supported is interposed between the antenna and the sensor. The sensed electromagnetic field strength provides a sensitive measure of the amount of microwave energy not absorbed by the food load but which flows around and/or through the food. The sensed electromagnetic field strength provides an indication of conditions within the food being cooked, particularly as to the moisture content of the food. The progress of cooking in the microwave oven is monitored by this arrangement; however, this apparatus represents an elaborate and expensive system for determining the cooking progress. Furthermore, the food load must be located beneath the antenna in order to provide accurate temperature indications.
In addition, several other types of cooking progress or cooking doneness indicators are presently being marketed for use on containers to be placed in microwave ovens. One such indicator is manufactured by the Armour Corporation, which provides a container having a liquid crystal display which changes color in response to the heat generated within the container by the food. This indicator is permanently attached to the container and is not reusable, thereby rendering the container impractical if the temperature indicator is required for repeated usage. Once the color changes, the indicator is non-reusable since the display never returns to its original color.
Other known indicators include two types manufactured by Omega Corporation, most notably the model R5 strip type indicator and the series of RRN models. The model R5 indicators comprise a strip of five different color temperature bands covering a range of 50.degree. to 70.degree. C. in 5.degree. C. increments. These adhesive backed strips may be applied to various surfaces and are temperature-reversible such that, as the temperature increases, the color changes for various temperature levels, and as the temperature decreases, the bands revert to their original colors. The RRN models are circular labels which may be attached to various surfaces, and also change color as the temperature increases and revert to their original color as the temperature decreases. A disadvantage of these type of indicators lies in the type of adhesive and the over-laminations required to both secure the labels to various surfaces and to provide reusability with regard to washing and overheating. The adhesive must be strong enough to prevent loosening during overheating and washing conditions, yet must be thin enough to allow for accurate temperature measurement. Furthermore, the adhesive must be compatible with the surfaces on which the label would be used so as not to peel off inadvertently.